1. Field of the Invention
The present invention relates to a semiconductor microdevice used for an acceleration sensor, an angular acceleration sensor, an electrostatic actuator or the like.
2. Description of the Background Art
FIG. 7 is a cross-sectional view which shows a structure of a conventional semiconductor microdevice 70. Semiconductor microdevice 70 is packaged so as to realize predetermined functions and operations. Semiconductor microdevice 70 is provided with two types of chips on a die pad 13 in a package. To be specific, the chips are a microstructure chip 71 provided on a chip substrate 74 and an application specific IC chip (to be referred to as “ASIC” (Application Specific Integrated Circuit) hereinafter) 72. These chips are appropriately selected according to the purpose of semiconductor microdevice 70. Microstructure chip 71 is electrically connected to chip substrate 74. Chip substrate 74 is connected to ASIC 72 by a bonding wire 75. ASIC 72 is also connected to a lead 17 a bonding wire 76. By connecting lead 17 to a circuit or the like outside of the package, power is supplied to microstructure chip 71 and ASIC 72 to enable these chips 71 and 72 to operate. It is noted that die pad 13 and chip substrate 74 are fixed to each other and die pad 13 and ASIC 72 are fixed to each other through resin layers 78, respectively.
There has been known hitherto a capacitive type inertial sensor made by a semiconductor micromachining technique as a sensor for detecting acceleration or the like. FIGS. 8A and 8B show a microstructure chip 80 used in the capacitive type inertial sensor. To be specific, FIG. 8A is a top view of microstructure chip 80 and FIG. 8B is a cross-sectional view of microstructure chip 80.
Microstructure chip 80 detects the capacitance of a capacitor formed by electrodes 82 and 83 suspended on a silicon substrate 81. Electrode 82 is a fixed electrode which is fixed to substrate 81. Electrode 83 is a movable electrode which can be moved relative to substrate 81 according to an inertial force. Movable electrode 83 is formed as one structure and supported by the silicon substrate 81 by beams 84. Since electrode 83 is movable relative to electrode 82, the position of electrode 83 is changed according to acceleration and the distance between electrodes 82 and 83, i.e., the capacitance of the capacitor is, therefore, changed. By detecting the change of the capacitance of the capacitor, it is possible to obtain the acceleration of an object, to which the capacitor is attached.
Since the change of the capacitance is very small (e.g., 1 pF), microstructure chip 80 is susceptible to external noise such as static electricity or radio wave. For that reason, a semiconductor microdevice (an acceleration sensor) employing conventional microstructure 80 is low in reliability.